Device for improving the security of aircraft in visual flight regime

ABSTRACT

Measurements of the instantaneous position and speed of an aircraft are incorporated in co-operation messages regularly broadcast to other aircraft by radio. The device fitted on the aircraft, flying under visual flight rules, analyzes the data contained in the co-operation messages received from other aircraft and its own trajectory in order to identify potential risk of collision. The pilot is alerted to these potential risks of collision, advantageously in voice form, in messages identifying the direction in which the other aircraft is visible as well as an estimation of the time remaining until the potential collision.

BACKGROUND OF THE INVENTION

The present invention relates to a device for preventing collision risksbetween aircraft operating under VFR (Visual Flight Rules).

The invention is adapted to the piloting rules used by VFR pilots, thebasic principle of which is: see and be seen. In effect, the reason mostcommonly found in reporting of accidents involving collision is theinability of the pilot to see the other aircraft in time. It istherefore desirable to improve the safety of all aircraft flying undervisual flight rules without running the risk of changing the rules,customs and habits of pilots who fly under VFR.

To this end, account needs to be taken of the specific aeronauticalconstraints on light VFR aviation. Gliders in particular are subjectedto the most stringent constraints because:

available energy is limited to the use of one additional 12 V/6 A.hbattery;

the space available is in the order of a few cubic decimeters;

the maximum mass per square centimeter and the total mass are restricteddue to the need to comply with certification constraints;

the addition of external antennas to the fuselage would be detrimental,as would any change in the aerodynamics of the craft. A system thatwould satisfy these constraints will generally be suitable to all theother categories of aircraft flying under VFR, in particular lightaircraft, helicopters, ULM . . .

A number of anti-collision devices currently exist, co-operative orautonomous, which are used in aeronautical applications.

Two categories of known autonomous devices may be cited:

1) Passive devices, which are generally optical and of the infraredsensor type, offer high performance in military applications where thetargets have adapted infrared signatures.

2) Active devices, of the airborne radar type, require transmission andreception of a radar wave. The bandwidth of the signal determines thedistance resolution performance. Other criteria, such as wavelength,determine the size of transmission and reception antennas.

Five categories may be cited among the co-operative devices:

1) Some devices seek to increase the aircraft visibility (anti-collisionlights, colored stripes painted or adhered to gliders, . . . ). Thesedevices generally depend on the type of aircraft on which they are used.They do not significantly increase the pilot's ability to look in theright place in good time.

2) Other devices are designed to optically detect the flashinganti-collision lights of aircraft. However, these lights are notprovided on all VFR aircraft. Secondly, the range of these devices islimited (approximately 1 km), which seems inadequate given the typicalspeeds at which light aircraft travel and the average reaction times.

3) Systems of the “TCAS” type (Traffic alert and Collision AvoidanceSystems) require a substantial source of energy and the installation oftwo sets of transmitting and receiving antenna respectively on the upperand lower parts of the aircraft. They operate as a real on-boardsecondary radar. They are only able to detect aircraft fitted withoperating transponders, which is not usually the case with VFR aircraft.Some of these systems use trajectographic algorithms to suggestavoidance maneuvers to the pilots. TCAS II, in particular, becamecompulsory in the USA in 1993 for all craft having more than thirtyseats.

4) Variants of TCAS require the support of secondary air control radarto listen to the transponders of neighboring aircraft when interrogated.This secondary radar support is not available all over the world andmakes the system dependent on ground installations.

5) Radio-electric devices of the beacon type may incorporate aposition-detecting means, a data transmitter and receiver, a unit forcomputing potential conflicts and a display device for the pilot. Anexample of such a device is described in U.S. Pat. No. 4,835,537. Likethe TCAS, these devices do not seem particularly well suited to VFRflight conditions since they require the use of display screens on whichrelatively complex information is presented, some of which relates toaircraft presenting virtually no risk of collision. The pilot'sattention is somewhat distracted by the instrument, which goes againstthe very principle of the visual flight rules.

Therefore, no collision prevention device is available today which isparticularly adapted to cover specific aspects inherent in the safety ofall aircraft types flying under VFR rules. In particular, there is nodevice capable of taking account of complex trajectories such as thoseof gliders moving in ascending spirals, which cause a considerablenumber of accidents.

The purpose of the invention is to propose anti-collision devices of theco-operative type which are especially well suited to VFR conditions.

SUMMARY OF THE INVENTION

Accordingly, the invention proposes a device to assist piloting undervisual flight rules, to be installed on board an aircraft andcomprising:

measuring means for estimating at least the instantaneous position andvelocity vector of the aircraft;

a radio transceiver for broadcasting co-operation messages containingparameters representing at least the estimated instantaneous positionand velocity vector of the aircraft and for receiving similarco-operation messages broadcast by other aircraft;

means for analyzing the co-operation messages received by the radiotransceiver and data output by the measuring means, to identifypotential risks of collision with other aircraft; and

a man-machine interface to alert the aircraft pilot to potential risksof collision identified by the analysis means.

According to the invention, the analysis means are arranged to performthe following operations at successive instants of analysis:

sub-dividing an analysis period, commencing at the current instant ofanalysis, into a series of consecutive time intervals;

for each of said time intervals, determining a protected volume on thebasis of different possible future positions of the aircraft derivedfrom the data output by the measuring means;

extrapolating the trajectory of each other aircraft from which aco-operation message is received, on the basis of the parameterscontained in said co-operation message, so as to estimate possiblepositions of said other aircraft in the time intervals of the series;and

if a condition is satisfied whereby an estimated possible position ofanother aircraft in one of said time intervals in the series is locatedwithin the protected volume determined for said time intervals,controlling the man-machine interface to issue the pilot with asignaling message indicating the direction in which said other aircraftis located at the current instant of analysis.

Being co-operative in nature, the device is common to all aircraft whichfly under VFR rules, including gliders in particular, and is operated apriori all over the world without the constraints inherent in any othercontrol mechanisms or dependent on ground-based installations.

The device can operate to an accuracy that will enable compliance withthe average reaction time of a VFR pilot. According to some studies,this time has been estimated at 12.5 seconds between the instant atwhich the pilot sees the other aircraft and the instant at which heavoids it, including sighting the object (0.1 s), recognizing it (1 s),realization of the certainty of collision (5 s), the decision to turn (4s), the muscular reaction (0.4 s) and the average response time of alight aircraft or glider (2 s).

The nature and operation of the device do not require to change therules, customs and habits of pilots flying under VFR, or imposeadditional workload on them.

Preferably, the signaling message also indicates the time remaininguntil the first of the time intervals in the analysis period for whichsaid condition is fulfilled. This will enable the pilot to assess howurgently he needs to act.

In order to facilitate visual location of the other aircraft, thesignaling messages advantageously include an indication of an apparentsize of the other aircraft, determined on the basis of real sizeindications included in the co-operation messages and of the distancebetween the two aircraft.

Preferably, the man-machine interface comprises means for issuingsignaling messages in the form of voice messages. This being the case,the device does not divert the visual attention of the pilot towards theinterior of the cockpit, unlike most anti-collision systems designed forlight aircraft, which use a display screen to provide the pilot with agraphical presentation of the traffic and/or potentially conflictingaircraft nearby. The structure of the signaling voice message may be inaccordance with a specific phraseology, comparable with conventionalradio telephone messages, e.g.: “Traffic, 15 seconds, 11 o'clock, 10°,size 2”. Thus, a pilot used to receiving such information messages willbe able to locate the aircraft posing a potential danger very rapidly.He will then be able to take the right decisions to avoid collision inview of the situation.

It will be left to the pilot to assess and choose the best avoidancemaneuver, on the basis of the information supplied by the device and incompliance with the VFR rules which he must apply. Accordingly, theprinciple of the visual flight rules based on observing the surroundingspace for 90% of the time will be satisfied.

The nature of the device and the algorithm used to process the availabledata enable a good adaptation to the different categories of aircraft inquestion, taking account of their speed, the way they move, their meandistribution density, the available space and the available on-boardenergy sources.

In a preferred embodiment of the device, the measuring means arearranged to estimate the instantaneous turning radius of the aircraft ina horizontal plane, on which the possible future positions of theaircraft depend, which the analysis means use as a basis for determiningthe protected volumes. By thus adapting the relevant protected volumesto the instantaneous turning radius of the aircraft, the effective risksof collision are better taken account of.

The duration of the analysis period is advantageously selected as anincreasing function of the instantaneous turning radius if the latter isestimated by the measuring means, for example as the minimum between areference period (or maximum warning time) and a time proportional tothe ratio between the instantaneous turning radius estimated by themeasuring means and the modulus of the projection on the horizontalplane of the instantaneous velocity vector estimated by the measuringmeans.

This limits the amount of information presented to the pilot, who isnegotiating the turn, so as not to distract his attention, by avoidingissuing alerts to aircraft which are too far away to pose a real risk ofcollision given the instantaneous velocity and turning radius. Thisadvantage is particularly useful in the case of gliders which have asmall turning radius when in an ascending spiral and are oftensurrounded by other gliders which should be signalled in due course.

The device is preferably adjustable by the pilot regarding the maximumcollision risk warning time. This will give him additional comfortcorresponding to the reaction time which he prefers to allow himselfdepending on his personal capabilities, the average quantity ofinformation which he would like to be delivered and the current flightphase (take-off, cruise, approach . . . ).

The nature of the device enables a relatively simple implementation andinexpensive manufacture. It may therefore be fitted systematically bysmall aircraft owners, unlike some of the other, relatively expensivedevices. Widespread use of this device can thus be achieved, a conditioncrucial to enhancing flight safety.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates examples of signaling messages issued by a deviceaccording to the invention to pilots flying aircraft under VFR rules;

FIG. 2 is an overall diagram of a device as proposed by the invention;

FIG. 3 is a flow chart setting out the operations performed by ananalysis module of the device illustrated in FIG. 2;

FIG. 4 is a perspective view of an elementary protected volume definedaround the aircraft; and

FIGS. 5 to 7 are schematic views in elevation, from above and inperspective of a protected volume defined for a given period of time.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 illustrates the conditions under which the present invention isapplied, depicting two aircraft 10, 10′ flying in proximity to oneanother in visual flight. Each of the aircraft is fitted with a deviceaccording to the invention, described in more detail below, whichenables its pilot to be issued with information, in the form of voicemessages, which will make it easier to visually locate the otheraircraft should a risk of collision with it arise within a certain zoneZ.

A signaling voice message of this type is issued in a format similar tothat which might be given by a co-pilot, making it easier for the pilotto interpret. In the illustration given in FIG. 1, this information ismade up of five elements:

the first (“Traffic”) indicates to the pilot the nature of the message,to draw his attention to the fact that he should be in sight of anotheraircraft;

the second (“20 seconds”) provides an estimation of the time remainingto the instant of a potential collision within the zone Z;

the third and fourth elements (“3 o'clock, elevation plus 5” for theaircraft 10, “11 o'clock, elevation minus 5” for the aircraft 10′)indicate, within a co-ordinate system linked to the aircraft, thegeneral direction in which the pilot should look in order to locate theother aircraft, i.e. the direction corresponding to the broken line inFIG. 1. This direction is indicated with the aid of an azimuth angle φexpressed, in a usual manner, in the form of the time indicated by thesmall needle of a clock dial, and an elevation angle expressed indegrees relative to the horizon line;

the fifth element (“size 1”) is an indication of the apparent size ofthe other aircraft, to make it easier to locate.

In order to work out such signaling messages, each aircraft 10 is fittedwith a device comprising:

measuring means for estimating instantaneous parameters x, y, z, V_(x),V_(y), V_(z), R of the trajectory of the aircraft 10 fitted with thedevice, in a co-ordinate system common to all the aircraft;

a radio transceiver for broadcasting co-operation messages incorporatingsome of the estimated instantaneous parameters for the aircraft 10, andfor receiving similar co-operation messages broadcast by other aircraft10′ located within range of the aircraft 10;

means for analyzing collisions risks on the basis of the trajectoryparameters estimated for the aircraft 10 and trajectory parametersobtained from the co-operation messages received by the radiotransceiver;

a man-machine interface for issuing the signaling messages based on theresult of the collision risk analysis.

In the device illustrated in FIG. 2, the measuring means pick uppositioning reference signals output by a satellite constellation, suchas GPS signals (Global Positioning System). They incorporate a GPSantenna 12 and an associated receiver 13 which processes the GPS signalsto derive therefrom the instantaneous position of the aircraft 10 in theform of its three co-ordinates x, y, z within a co-ordinate systemindependent of the aircraft. By way of example, the x co-ordinate maycorrespond to longitude, the y co-ordinate to latitude and the zco-ordinate to altitude, the plane xOy being a horizontal plane. Forcertain parameters, the GPS measurements might be completed bymeasurements taken by on-board sensors of the aircraft (for example analtimeter in the case of the parameter z).

In the architecture illustrated in FIG. 2, the device consists of acomputer 15, such as a microprocessor having a working memory and aprogram memory. In the block diagram of FIG. 2, the computer 15 hasthree modules 16,17,18 which, in practice, may be software modules, i.e.program elements run by the microprocessor.

The module 16 derives the position parameters x,y,z output by the GPSreceiver 13 in order to estimate the co-ordinates V_(x),V_(y),V_(z) ofthe velocity vector {right arrow over (V)} of the aircraft 10 within thesame co-ordinate system, as well as the instantaneous turning radius Rof the aircraft 10 in the horizontal plane xOy. Two successive GPSmeasurements enable the speed vector {right arrow over (V)} to beestimated and three successive measurements enable the turning radius Rto be estimated.

The module 17, firstly, formats the co-operation message broadcast bythe radio transceiver 20 and, secondly decodes the co-operation messagesreceived from other aircraft 10′. Each co-operation message correspondsto a data block, the size of which may be, e.g., 90 bits. An example ofhow the bits are distributed in such a block is given in Table I.

TABLE I Number Sign Data type of bits bits Accuracy range Longitude x 231 10^(−4°) ±180° Latitude y 22 1 10^(−4°) ±90° Altitude z 13 0 4 m  32767 m x velocity 8 1 1 m/s +255 m/s y velocity 8 1 1 m/s ±255 m/s zvelocity 6 1 0.5 m/s   +31 m/s Aircraft size T 2 0 1 4 Checksum 3 Total90

In table I, the first six rows correspond to the instantaneous positionand velocity x,y,z, V_(x),V_(y),V_(z) of the aircraft. The size T of theaircraft is an indication which may assume four values and representsthe real size of the aircraft (for example, proportional to its span).The checksum (CRC) is made up of three redundancy bits enabling theprocessing module 17 to detect any transmission errors in a receivedco-operation message.

In view of the difficulty in obtaining a frequency bandwidth for acommunication channel, the protocol applied by the radio transceiver 20preferably uses a common frequency channel to be shared by all theparticipants.

The protocol used to communicate is of the time-division multiple accesstype (TDMA) where each aircraft transmits on a common carrier frequency.The channel can be optimized by an appropriate protocol of the ALOHAtype, whereby the different participants transmit in turn during givenperiods, synchronized by GPS timing. A protocol of this type isdescribed in French patent 2,708,124 and its U.S. counterpart U.S. Pat.No. 5,544,075, incorporated herein by reference.

The frequency channel is divided into reproducible periods T_(c), eachperiod comprising a number N_(c) of equally distributed time slots. Inorder to transmit, a time slot is accessed by identifying free timeslots during a listening period of at least T_(c) seconds and thenrandomly choosing one of the slots identified as being free. This methodincludes a channel access procedure (listening and then choosing a freeslot), a phase of occupying the chosen slot and a procedure of releasingthe slot.

A practical way of optimizing transmission time is to adapt the timeperiods T_(c), the number of slots N_(c) and the data format to thepresent application. The protocol may be adapted to the data to betransmitted as follows: a period T_(c)=1 second is chosen in order toallow the locating data derived from the GPS signals to be sufficientlyrefreshed; the number of time slots is N_(c)=100 which, for the timeT_(c)=1 second, leaves a period of 0.01 second for transmission of thepayload data. Accordingly, this number N_(c) is judiciously chosen so asto be higher than the number of VFR aircraft likely to be encountered inthe zone covered by the radio transceiver 20 in extreme cases, forexample during glider competitions.

Preferably, for a transmission rate of 9600 baud (value typical ofcommercially available modems which, in VHF, comply with the authorizedbandwidth of ±12.5 kHz), the maximum size of a data block will thereforebe 96 bits per aircraft.

With this transmission rate of 9600 baud, if occupancy of the channel isoptimized by allocating a 0.01 second time slot to each aircraft, the 90bit block of table I can be transmitted in 9.375 ms, which leaves a freetime of 625 μs between consecutive messages to enable the transceivers20 to switch from transmission to reception and to be able to insertlistening ranges within the occupied ranges in accordance with themethod described in French patent 2,708,124.

The radio transceiver 20 picks up the co-operation messages broadcast bythe aircraft 10′ located within radio range of the aircraft 10(typically a few kilometers) and extracts the positioning informationx′,y′,z′ of these aircraft 10′, their velocity informationV′_(x),V′_(y),V′_(z) and their real size details T.

This information, along with that from the GPS receiver 13 and thededuction module 16, are processed by the analysis module 18 to evaluatethe collision risks. This analysis may be in accordance with theprocedure shown in FIG. 3, which is run at successive instants ofanalysis t_(p) separated by a period T_(c) (t_(p+1)=T_(p)+T_(c)) if thenumber NA of co-operation messages received from other aircraft 10′ overthe last period T_(c) is greater than 0.

For each aircraft 10, an elementary protected volume is defined, whichis fixed relative to the aircraft. The dimensions of this volume must becompatible with the GPS locating accuracy, i.e. a few tens of meters. Asillustrated in FIG. 4, this elementary protected volume Pr may be in theshape of a rectangular parallelepiped, of a length L_(a) in thelongitudinal direction of the aircraft 10, a width L_(b) along the spanof the aircraft 10 and a height L_(c). By way of example, if L_(a)=200 m(with protection as a priority in the direction of displacement),L_(b)=100 m (chosen in relation to the relative accuracy of the GPS) andL_(c)=50 m (the approach speed is lower in the vertical direction apriori).

Starting at each instant of analysis t_(p), the module 18 defines ananalysis period of a duration D. Advantageously, this duration D is afunction of the instantaneous turning radius R and the velocity V_(h) ofthe aircraft 10 in the horizontal plane xOy, i.e. V_(h)={square rootover (V_(x) ²+L +V_(y) ²+L )}.

In the example illustrated in FIG. 3 (step 40 ), the duration D of theanalysis period is an increasing function of the instantaneous turningradius R supplied by the deduction module 16, namely the minimum betweena reference period D_(max) (or maximum warning time) and a period equalto $\frac{2\quad \pi \quad R}{V_{h}}.$

This latter period $\frac{2\quad \pi \quad R}{V_{h}}$

is equal to the time needed for the aircraft 10 to make a completerevolution, as seen in the horizontal plane, if it maintains itsinstantaneous velocity and turning radius. The fact of limiting theperiod D to this time (or to a time proportional to it) allows thequantity of information that will be issued to the pilot about thetypical instantaneous trajectories of all the VFR aircraft (includinggliders in ascending spiral) to be limited.

At step 41 of the analysis procedure, the module 18 sub-divides theanalysis period [t_(p), t_(p)+D] into a series of N consecutive timeintervals. Each of these time intervals has a duration${\delta = \frac{L_{a}}{V_{h}}},$

the number N being an integer equal to or immediately higher than theratio $\frac{D}{\delta}.$

Then, for each of the time intervals from n=0 to n=N−1, the module 18determines a protected volume Pr(n) on the basis of different possiblefuture positions of the aircraft 10 deduced from the trajectoryparameters x,y,z,V_(x),V_(y),V_(z),R, and examines whether otheraircraft are likely to enter this volume within the time intervalconsidered. In the example illustrated in FIG. 3, these operations areperformed in a loop indexed by the integer n (initialized at n=0 at step42 and incremented by one unit at step 43 after these operations havebeen run if n<N−1 at test 44 ).

The protected volume Pr(n) is defined in step 45 based on the elementaryvolume Pr of FIG. 4. This protected volume Pr(n) is, for example, builtusing the following procedure illustrated in FIGS. 5 to 7.

Viewed as projected in a horizontal plane (FIG. 6), the possible futurepositions M_(i,j)(n) of the aircraft 10 taken into account to determinethe protected volume Pr(n) are contained, on the one hand, between thestraight half-curve Δ defined by the position x,y,z and the velocityvector {right arrow over (V)} determined on the basis of the last GPSmeasurements and, on the other hand, the convex side of the arc Adefined by the position x,y,z, the velocity vector {right arrow over(V)} and the instantaneous turning radius R supplied by the module 16.Projecting into this horizontal plane, a number 1+I(n) possiblepositions M_(i,0)(n) are taken into account for the time interval n,associated respectively with velocity vectors {right arrow over(V)}_(i,0)(n). The number I(n) depends on the relative values of n.δ, R,V_(h), L_(a) and L_(b). The position M_(i,0)(n) and velocity vector{right arrow over (V)}_(i,0)(n) are computed for 0≦i≦I(n), as beingthose which the aircraft 10 would have at the instant t_(p)+n.δ if itwere to maintain its horizontal speed V_(h)=∥{right arrow over(V)}_(i,0)(n)∥ on a trajectory with a constant radius of curvature equalto $\frac{R \cdot {I(n)}}{i}.$

Viewed in projection in a vertical plane, the possible positionsM_(i.j)(n) of the aircraft for the period of time n are between thestraight half-curve Δ′ defined by the position x,y,z and theinstantaneous velocity vector {right arrow over (V)} derived from theGPS measurements on the one hand and the straight half-curve Δ″ definedby the position x,y,z and by the horizontal direction on the other (FIG.5). Each of the J(n) points M_(i,j)(n) for 1≦j≦J(n) (the number J(n)depends on the relative values of n.δ, V_(z), L_(a) and L_(c)) isderived from the point M_(i,0)(n) by assuming that the verticalcomponent of the associated velocity vector {right arrow over(V)}_(i,j)(n) is constant from t_(p) to t_(p)+n.δ and equal to$\frac{V_{z^{\prime}}j}{J(n)},$

the horizontal component of {right arrow over (V)}_(i,j)(n) being {rightarrow over (V)}_(i,0)(n).

By locating the elementary protected volume Pr in relation to thepossible future position M_(i,j)(n) and by orienting this elementaryvolume in accordance with the associated velocity vector {right arrowover (V)}_(i,j)(n), an elementary volume is obtained, noted Pr_(i,j)(n).The protected volume Pr(n) determined at step 45 for the time interval ncorresponds to the merger of these elementary volumes Pr_(i,j)(n) for0≦i≦I(n) and 0≦j≦J(n). This volume Pr(n) is approximately of the shapeillustrated in FIG. 7.

Having thus defined the protected volume Pr(n) at step 45, the analysismodule 18 examines whether one of the aircraft 10′ from which aco-operation message has been received is likely to enter this protectedvolume Pr(n) during the time interval n. In the example illustratedhere, this is performed in a loop, the iterations of which arecontrolled by an index k denoting the co-operation messages received or,in an equivalent manner, the aircraft which have broadcast thesemessages (initialized by k=1 at step 46 and incremented by one unit atstep 47 at the end of the iteration). A bit b(k) is associated with eachco-operation message received, corresponding to an aircraft k,indicating whether this aircraft has already been considered as posing acollision risk in the analysis performed from instant t_(p). Beforeproceeding with this analysis, all the bits b(k) are initialized to 0.The number NAS, initialized to 0, denotes the number of aircraft whichhave been considered as posing a risk of collision.

If the index k is lower than or equal to the number NA of aircraftdetected (test 48), and if the aircraft k has not already beenidentified as posing a collision risk (b(k)=0 in the test 49), theanalysis module 18 extrapolates the trajectory of the aircraft k overthe time interval n at step 50. To this end, it determines the segmentS_(k)(n) which the aircraft k will describe in the time interval n if itmaintains its velocity vector {right arrow over (V)}, the co-ordinatesof which are provided by the processing module 17, as well as itsinstantaneous position x′,y′,z′ also provided by the processing module17.

The ends of this segment S_(k)(n) are the two points of co-ordinates${\begin{pmatrix}x^{\prime} \\y^{\prime} \\z^{\prime}\end{pmatrix} + {{n \cdot \delta \cdot {\overset{\rightarrow}{V}}^{\prime}}\quad {and}\quad {{\begin{pmatrix}x^{\prime} \\y^{\prime} \\z^{\prime}\end{pmatrix} + {\left( {n + 1} \right) \cdot \delta \cdot {\overset{\rightarrow}{V}}^{\prime}}}\quad}}}$

in the Cartesian system common to the aircraft.

At step 51, the risk of collision during the interval n is estimated byexamining whether at least a part of the segment S_(k)(n) is locatedinside the protected volume Pr(n). The fact of having defined theprotected volume Pr(n) as a combination of parallelepipedic elementaryprotected volumes at step 45 considerably facilitates the check made atstep 51, given that it is very easy to determine whether theintersection between a segment and a rectangular parallelepiped is emptyor not.

If step 51 reveals a risk of collision (intersection not empty), themodule 18 computes, at step 52, the azimuth angle φ and the elevationangle α corresponding to the direction in which the aircraft kpotentially in conflict is visible from the aircraft 10 at the instantt_(p). These angles φ,α are simply determined by a change of co-ordinatesystem from the coordinates x,y,z and x′,y′,z′ of the two aircraft.

At step 52, the module 18 also evaluates the apparent size TA of theother aircraft k from the ratio $\frac{T}{Dist}$

between the real size T of the aircraft k, obtained by the module 17 inthe corresponding co-operation message and the distance Dist={squareroot over ((x′−x)²+L +(y′−y)²+L +(z′−z)²+L )} between the two aircraft.

At step 53, the analysis module 18 informs the man-machine interface 21of the device that a signaling message must be issued, indicating: (i)that a time n.δ remains before a potential collision; (ii) the azimuthand elevation angles φ,α computed at step 52, and (ii) the apparent sizeTA computed at step 52. The module 18 changes the value of the bit b(k)and increases the number NAS by one unit at step 54. If the number NASis equal to the number of aircraft NA, then all the aircraft for which aco-operation message was received have already been the object of asignaling message, so that the analysis from instant t_(p) isterminated.

The iteration k of the loop terminates with the incrementation at step47 if b(k)=1 at step 49 (aircraft already signaled), if S_(k)(n)∩Pr(n)=Øat step 51 (collision unlikely) or if NAS<NA at step 55. When all theaircraft have been reviewed for the time interval n (k>NA at step 48 ),the analysis module 18 moves on to step 44 to study the next timeinterval n, or to terminate the analysis from instant t_(p).

The man-machine interface (FIG. 2) comprises a voice synthesis module 22receiving the information supplied by the analysis module 18 at step 53.This module 22, which is of a known design, works out the voicesignaling messages and controls a loudspeaker or an earpiece 23 so thatthese voice messages are issued to the pilot.

The man-machine interface 21 also has a module 25 for regulatingparameters enabling the pilot, using a unit such as a keyboard 26, toselect the value of certain parameters of the trajectographic algorithmand/or the collision risk assessment algorithm run by the computer 15.

In particular, it is practical to be able to select the maximum warningtime D_(max) (used at step 40). The assisting device can thus issue thepilot with more or less information by exploring a more or lessextensive zone in front of the aircraft, depending on the experience ofthe pilot or the mean reaction time which he estimates he has or on thecurrent flight phase (take-off, cruise, approach . . . ).

It should be pointed out that the collision risk analysis procedureexplained above with reference to FIGS. 3 to 7 is only one example ofsuch a procedure which might be implemented using a device according tothe invention. Various alternatives are possible at different steps ofthe procedure.

For example, the extrapolation of the trajectory of other aircraft atstep 50 might consist, rather than determining a segment computed byextending the velocity vector of the other aircraft, in computing avolume taking account of a potential variation in the velocity vector ofthe other aircraft within a certain range. However, this would increasethe required amount of computation.

It would be conceivable to use other types of collision risk analysis indifferent aircraft, with a relatively simple analysis for inexpensiveversions of the device and more sophisticated analysis in otherversions. It would also be conceivable for different types of analysisto be provided in a same device, which could then be selected by thepilot with the aid of the unit 25 of the man-machine interface 21.

It is essentially at the level of broadcasting co-operation messagesthat a certain uniformity must exist between the different VFR aircraftso that these co-operation messages are fully understood by each ofthem.

The fact that the device incorporates a GPS receiver 13 and a radiotransceiver 20 advantageously permits that it fulfils other functions.

For example, if the aircraft 10 is fitted with a navigation aid system30, the positioning data x,y,z issued by the GPS receiver 13 andoptionally the velocity or curvature data V_(x),V_(y),V_(z),R producedby the derivation module 16 may be applied to this system 30 so that itcan process them.

The positioning data x,y,z regularly issued by the GPS receiver 13 mayalso be applied to a recorder 31 containing a certain number of memorylocations managed on a first in-first out (FIFO) basis in order to storea certain number of estimated positions of the aircraft up to thecurrent instant with the aid of GPS measurements. The recorder 31co-operates with an impact sensor 32 such as an accelerometer. In theevent of impact, the data stored in the recorder 31 is locked. Therecorder 31 therefore assumes the role of a black box enabling thecourse of the aircraft prior to impact to be tracked.

The positioning data x,y,z may also be applied to a unit 33 controllinga distress beacon. This unit 33 controls the radio transceiver 20 inresponse to an impact detected by the accelerometer 32 or in response toa manual control 34 activated by the pilot so that a distress messageincorporating the latest data x,y,z can be issued, for example on thefrequency 121.5 MHz. The fact that this distress message incorporatesthe last position of the aircraft evaluated by the GPS receiver 13greatly facilitates the task of locating the aircraft in the event ofdifficulty, relative to the conventional distress beacons which areusually fixed by goniometry.

What is claimed is:
 1. Device to assist piloting under visual flightrules, to be installed on board an aircraft and comprising: measuringmeans for estimating at least an instantaneous position and a velocityvector of the aircraft; a radio transceiver for broadcastingco-operation messages containing parameters representing at least theestimated instantaneous position and velocity vector of the aircraft andfor receiving similar co-operation messages broadcast from otheraircraft; means for analyzing the co-operation messages received by theradio transceiver and data output by the measuring means, to identifypotential risks of collision with other aircraft; and a man-machineinterface to alert the aircraft pilot to potential risks of collisionidentified by the analysis means, the analysis means are arranged toperform the following operations at successive instants of analysis:sub-dividing an analysis period, commencing at the current instant ofanalysis, into a series of consecutive time intervals; for each of saidtime intervals, determining a protected volume on the basis of differentpossible future positions of the aircraft derived from the data outputby the measuring means; extrapolating the trajectory of each otheraircraft from which a co-operation message is received, on the basis ofthe parameters contained in said co-operation message, so as to estimatepossible positions of said other aircraft in the time intervals of theseries; and if a condition is satisfied whereby an estimated possibleposition of another aircraft in one of said time intervals in the seriesis located within the protected volume determined for said timeintervals, controlling the man-machine interface to issue the pilot witha signaling message indicating the direction in which said otheraircraft is located at the current instant of analysis.
 2. Deviceaccording to claim 1, wherein the signaling message further indicatesthe time remaining until the first of the time intervals in the analysisperiod for which said condition is fulfilled.
 3. Device according toclaim 1, wherein each co-operation message broadcast by the transceiverincludes an indication of a real size of the aircraft, and wherein eachsignaling message relating to another aircraft, from which aco-operation message is received, includes an indication of an apparentsize of said other aircraft determined on the basis of the real sizeindication included in the received co-operation message and of thedistance between the two aircraft derived from the position estimated bythe measuring means and from the position of the other aircraft includedin the received co-operation message.
 4. Device according to claim 1,wherein the man-machine interface comprises means for issuing thesignaling messages in the form of voice messages.
 5. Device according toclaim 3, wherein the signaling message further indicates the timeremaining until the first of the time intervals in the analysis periodfor which said condition is fulfilled, wherein the man-machine interfacecomprises means for issuing the signaling messages in the form of voicemessages, and wherein each signaling voice message relating to anotheraircraft comprises an indication of the nature of the message, saidremaining time, the direction in which said other aircraft is located,expressed by an azimuth angle and an elevation angle, and the indicationof apparent size.
 6. Device according to claim 1, wherein the measuringmeans comprise a receiver for positioning reference signals transmittedfrom a constellation of satellites.
 7. Device according to claim 1,wherein the radio transceiver is arranged to operate using atime-division multiple access protocol on a common carrier frequency,the co-operation message being transmitted in a time slot which is notoccupied by a co-operation message from another aircraft.
 8. Deviceaccording to claim 1, wherein the measuring means are arranged toestimate an instantaneous turning radius of the aircraft in a horizontalplane and wherein the future possible positions of the aircraft, whichthe analysis means uses as a basis for determining the protectedvolumes, depend on the instantaneous turning radius estimated by themeasuring means.
 9. Device according to claim 8, wherein the analysisperiod has a duration chosen as an increasing function of theinstantaneous turning radius estimated by the measuring means. 10.Device according to claim 9, wherein the duration of the analysis periodis chosen as the minimum between a reference duration and a durationproportional to a ratio between the instantaneous turning radiusestimated by the measuring means and a modulus of a projection in thehorizontal plane of the instantaneous velocity vector estimated by themeasuring means.
 11. Device according to claim 10, wherein the referenceduration is adjustable via the man-machine interface.
 12. Deviceaccording to claim 8, wherein, viewed in projection on a horizontalplane, the future possible positions of the aircraft, which the analysismeans uses as a basis for determining the protected volumes, arecontained between a straight half-curve defined by the instantaneousposition and velocity vector estimated by the measuring means and aconvex side of an arc defined by the position, instantaneous velocityvector and turning radius estimated by the measuring means.
 13. Deviceaccording to claim 12, wherein, seen in a projection on a verticalplane, the possible future positions of the aircraft, which the analysismeans uses as a basis for determining the protected volumes, arecontained between a straight half-curve defined by the instantaneousposition and velocity vector estimated by the measuring means and astraight half-curve defined by the instantaneous position estimated bythe measuring means and by the horizontal direction.
 14. Deviceaccording to claim 1, wherein the analysis period has a duration at mostequal to a reference duration which is adjustable via the man-machineinterface.
 15. Device according to claim 1, wherein the radiotransceiver is arranged to broadcast a distress signal, incorporatingthe position of the aircraft estimated by the measuring means, inresponse to a manual command from a pilot or automatic detection of acollision.
 16. Device according to claim 1, wherein at least some of thedata output by the measuring means are further provided to a navigationaid system.
 17. Device according to claim 1, further comprising meansfor recording a number of positions of the aircraft estimated by themeasuring means up to a current instant, the recording being locked inresponse to automatic detection of a collision.